Move some of the changes I made to Special:Categories out of SpecialCategories.php...

Move some of the changes I made to Special:Categories out of SpecialCategories.php and into Pager.php.  Allow multiple possible sort orders for IndexPager, allow user override of sort direction for IndexPager, add extra links as appropriate for AlphabeticPager.  Some of this seems to duplicate TablePager logic; I'm not sure why that's a separate class to begin with.  Or why AlphabeticPager is called that, given that it's not necessarily alphabetic at all.  In fact all of these child classes seem to perform almost identical functions and it seems as though folding them into the IndexPager class would reduce code duplication.